The present system relates generally to a system for use on board a vehicle to supply the needs of the vehicle, and more particularly to an improved vehicle generation system having a brushless doubly-fed generator.
Other vehicle generator or alternator systems have been incorporated into vehicles to supply the electrical requirements of the vehicle and to supplement and recharge a DC battery on board the vehicle. For vehicles propelled by engines, typically the alternator rotor is coupled to and driven by the engine via linking means, such as an endless belt. The electrical output of the alternator is alternating current (AC) power which is generally converted to direct current (DC) power by a rectifier. The DC power is supplied via a DC bus to a variety of onboard electrical devices and to a DC battery for storage on board the vehicle.
For example, in the automotive industry, DC brush-type generators were used in cars until the early 1960's. The brush-type generators suffered from a variety of commutation problems which severely restricted the reliability of the automobile's power generation system. Since that time, the automotive industry has used and continually improved a claw pole Lundell-type alternator.
The Lundell alternator includes a rotor having a single coil embedded therein which, upon rotation, establishes a multipolar bidirectional field. The voltage produced by the Lundell alternator is regulated by controlling the current flow within the rotor coil. However, the magnetic field of the Lundell alternator must pass axially through the magnetic core and the shaft so that the output power may only be increased by increasing the rotor diameter and thus the overall size of the alternator.
Furthermore, eddy current losses in the rotor are increased due to the solid nature of the pole segments facing the air gap. The rotor eddy currents increase the temperature of the field coil which reduces the overall efficiency and output capability of the alternator. Further contributing to the losses of the Lundell alternator are the brushes required to transfer current from a regulator to the rotor coils.
In sum, typical automobile generators have losses resulting in efficiencies of 60% or less. Additionally, the requirement of brushes detracts from the overall reliability of a generator, limits the generator maximum speed, and increases the maintenance costs of the power generation system.
The onboard power generation requirements in the automotive field are continually increasing, particularly in the luxury car field. The vehicles of today typically have more than 40 motors, solenoids and actuators, such as the starter motor, power windows, blowers, seat and mirror adjusters, wipers and washers, and door and trunk locks, all of which are powered by the onboard power generation system. Many of today's cars have microprocessor-controlled fuel and ignition systems incorporating additional motors, actuators and solenoids, and which require a reliable power source to promote a fast response and precise control of these systems and devices. The automotive industry is also looking toward new electrically powered systems on board a vehicle, such as electric steering and braking, active suspension systems, heated windshields and electric air conditioning. It has been estimated that by the year 2000 the electric power consumption on board automobiles will be approximately three to five kilowatts.
In the face of these increasing automotive power generation requirements, it appears that the Lundell-type alternator may be nearing the end of its useful life in automotive applications. A single Lundell alternator would have to be significantly increased in size over the types conventionally used today to meet these increased power requirements. Alternatively, two Lundell alternators could be used on board, but would decrease the overall efficiency of the engine, decrease fuel economy and/or require a larger engine. Furthermore, the underhood space constraints of today's automobiles render both the larger Lundell alternator and the dual Lundell alternator system to be impractical alternatives to meet these increasing power needs.
A permanent magnet machine or an induction machine could be used as an automotive alternator, but each is far more costly than a Lundell-type alternator. For example, the permanent magnet machines require expensive exotic core materials. Furthermore, permanent magnet alternators and induction alternators each require costly controllers to provide suitable power for use in an automotive generation system.
Thus, a need exists for an improved power generation system for use on board vehicles, such as the luxury automobiles of the near future, which is not susceptible to the above limitations and disadvantages.